1. Field of the Invention
The present invention relates to a negative electrode active material and a secondary battery.
2. Discussion of Related Art
A secondary battery is a chemical cell that can be repeatedly charged and discharged using an electrochemical reaction. One representative example of the secondary battery is a lithium secondary battery. In a lithium secondary battery, for example, a carbon-based material such as graphite is used for a negative electrode, a lithium-containing compound is used for a positive electrode, and a lithium salt is used for an electrolyte.
Portable electronic equipment is provided with a device having high power consumption, such as a central processing unit (CPU) and a liquid crystal display device (LCD), and shows increasing power consumption with multifunctionality. Therefore, a lithium secondary battery frequently used as an electric power source of the electronic equipment is required to realize a high-capacity design. As a widely used negative electrode active material, however, graphite has a problem in that it is difficult to achieve a high charge/discharge capacity.
Accordingly, as an alternative to a carbon-based negative electrode active material, research on a metallic material that can introduce and deintercalate lithium ions has been conducted, and much attention has been paid to silicon (Si) and tin (Sn) as the alternative to the carbon-based negative electrode active material.
However, a cell using silicon (Si) as a negative electrode material has a problem in that an expansion in volume of the silicon (Si) may take place during a charge cycle when lithium ions transferred to the negative electrode are intercalated into the silicon (Si). Also, the lithium ions are deintercalated from the silicon (Si) during a discharge cycle, which leads to re-contraction of the silicon (Si). As a result, a metal is ground into a powder and a cycle property may be degraded as the charge/discharge cycle is repeatedly carried out.
Japanese Patent Laid-open Publication No. 2006-286312 disclosed a lithium secondary battery using a silicon thin film formed through a dry process as a negative electrode active material. In Japanese Patent Laid-open Publication No. 2006-286312, the silicon thin film has a structure which is separated in a main phase according to a short circuit formed in a thickness direction thereof. In Japanese Patent Laid-open Publication No. 2006-286312, there have been attempts conducted to reduce the expansion and contraction of silicon (Si) using a physical shape of a silicon thin film, but the productivity of the thin film is severely lowered, which makes it difficult to apply the thin film to mass production of cells.